In towed marine seismic exploration, a hydrophone array is towed behind a marine vessel near the sea surface. The hydrophones reside in multiple sensor cables commonly referred to as streamers. A seismic source, also towed near the sea surface, periodically emits acoustic energy. This acoustic energy travels downward through the sea, reflects off underlying geologic structures, and returns upward through the sea to the hydrophone array. The hydrophone array records the upward-traveling seismic acoustic-pressure waves from the seabed. The hydrophone recordings are later processed into seismic images of the underlying geologic structures.
Acoustic impedance is the ratio of pressure to particle velocity and is equal to the product of the density ρ and the speed of sound c in the acoustic medium, ρc. Reflections occur any time a change in acoustic impedance is encountered by the sound waves. The greater the change in acoustic impedance, the more the energy is reflected. Since the acoustic impedance of air and water differ greatly, the sea surface is a nearly perfect reflector of sound energy. After returning from the sea bottom or the target of interest, the energy is again reflected by the sea surface back toward the hydrophone array. Because a hydrophone has an omnidirectional spatial response, the hydrophone array records a ghost response, which is the seismic acoustic wave reflected from the sea surface and arriving delayed in time and reversed in polarity from the direct reflection. The ghost is a downward traveling seismic acoustic wave that, when added to the desired wave, detracts from the recorded seismic image.
The ghost produces a notch in the frequency spectrum of a hydrophone response at fnotch=c/2d, where c is the speed of sound and d is the hydrophone array depth. Seismic hydrophone arrays have been conventionally towed at depths of 10 meters or less. At a depth of 10 m, the notch frequency (fnotch) is 75 Hz. A frequency response extending beyond 100 Hz is required for high seismic-image resolution. Hydrophone arrays are therefore sometimes towed at shallower depths to improve the resolution of a seismic image.
The ghost-causing reflection can also continue to the sea bottom or other strong reflector and be reflected back up to again interfere with the desired reflections and degrade the image. These reflections are commonly referred to as multiples.
Towing at shallow depths is problematic because noise from the sea surface interferes with the desired seismic signals. Furthermore, circular water currents near the sea surface can cause increased flow noise at the streamer skin. These effects are worsened as weather deteriorates, sometimes causing the crew to discontinue operations until the weather improves. The deeper the tow, the less sea-surface noise and weather are factors. If the ghost-notch effects can be eliminated, it is desirable to tow at greater depths.
Ocean-bottom, or seabed, systems, in which the sensors are placed on the seabed, reject ghosts by a technique commonly known as p−z summation. In an acoustic wave, the pressure p is a scalar and the particle velocity u is a vector. A hydrophone records the seismic acoustic wave pressure p, with a positive omnidirectional spatial response. A vertically oriented geophone or accelerometer records the vertical component of the seismic acoustic wave particle velocity uz, with a positive response to up-going signals and a negative response to down-going signals. In p−z summation, the velocity signal is scaled by the acoustic impedance ρc of seawater and added to the pressure signal. If an accelerometer is used, its output can be integrated to obtain the velocity signal, or the hydrophone pressure signal can be differentiated so that it can better spectrally match the accelerometer. This produces a compound sensor that has full response to the upward traveling wave and zero response to the downward traveling wave to reject the ghost and multiples. One such method of signal conditioning and combination of signals to get a single de-ghosted trace is described in U.S. Pat. No. 6,539,308 to Monk et al. This and similar techniques work well when the acoustic particle-velocity sensor or accelerometer is not affected by unwanted motions due to factors not caused by the desired signal. Such unwanted accelerations are common in a seabed system deployed in a surf zone or area when there are strong bottom currents.
Recently there has been interest in using the combination of hydrophones and particle-motion sensors to reduce the ghost and multiple effects in a seismic streamer. Operating a particle-motion sensor in a seismic streamer presents a problem because the streamer experiences accelerations due to towing and sea-surface effects that are large compared to accelerations caused by the desired seismic reflections. Moreover, these unwanted accelerations are in the same spectral band as the desired seismic reflection response.
Seismic streamers and ocean-bottom seismic cables experience all roll angles from 0° to 360° and moderate pitch angles. To implement a vertically oriented geophone, ocean-bottom systems have used: (a) a gimbaled moving-coil geophone; (b) a 3-component, omni-tilt moving-coil geophone with external attitude sensing and computation external to the sensor to resolve the measurement relative to gravity; and (c) a 3-component, micro-electro-mechanical system (MEMS) accelerometer with internal attitude sensing and computation external to the sensor to resolve the measurement relative to gravity.
U.S. Pat. No. 7,167,413 to Rouquette uses an accelerometer acoustic-wave particle-motion sensor in a seismic streamer to reject the ghost-notch effect. Rouquette uses a mass-spring vibration isolation system to reduce the effect of cable dynamic motion on the accelerometer and a load-cell system to measure and reject the residual cable motion induced noise on the accelerometer. The Rouquette system relies on well-known mechanical relationships that do not remain constant with manufacturing tolerances, aging, and environmental conditions. Rouquette uses a signal processing adaptive algorithm to derive the relationship of the mass-spring system to the acceleration acting on the accelerometer in situ. Dynamic shaking of the accelerometer caused by turbulent flow of the acoustic medium past the sensor is treated the same as the cable dynamic motion and is removed from the acoustic-wave particle-motion measurement. Rouquette describes a complex mechanical and electronic system.
U.S. Pat. No. 7,239,577 to Tenghamn et al. describes an apparatus and method for rejecting the ghost notch using an acoustic-wave particle-velocity sensor. Tenghamn et al. teaches the use of a fluid-damped, gimbaled geophone. It is known in the art that the fluid encapsulating the geophone is chosen to provide damping of the sensor swinging on its gimbals. While not described in Tenghamn et al., it is known in the art that a mass-spring vibration-isolation system can reduce the effect of cable dynamics on the geophone response. But dynamic shaking of the geophone caused by turbulent flow of the acoustic medium past the sensor is not addressed in Tenghamn et al. Motion of the geophone caused by cable dynamics and by turbulent flow of the acoustic medium past the sensor is indistinguishable from acoustic-wave particle motion in the geophone response. The desired seismic-wave particle motion is obscured by cable dynamic motion and turbulent-flow-induced motion in Tenghamn et al.
U.S. Pat. No. 7,359,283 to Vaage et al. describes a method of combining pressure sensors and particle-motion sensors to address the impact of cable dynamic motion and turbulent flow on the particle-motion sensors. In this method, the response of the particle-motion sensor below a certain frequency f0 is not used, but only estimated from the pressure-sensor response and the known pressure-sensor depth. The frequencies rejected are those for which dynamic motion of the streamer is expected and for which turbulent flow of the acoustic medium past the sensor shakes the sensor. The estimated response has poor signal-to-noise ratio at the lower frequencies of interest. This rejection below a certain frequency is not optimal as it also rejects valid signals in an important low-frequency band where deep-target seismic data is likely to exist.
While the patents mentioned all describe methods to reject the ghost notch in a seismic streamer using multi-component acoustic-wave measurements, all fall short of adequately accounting for the effects of sensor-mount motion, sensor tow through the acoustic medium, and acoustic-medium motion on multi-component acoustic sensors. All also fall short of producing high-fidelity, sensed acoustic-wave components with good signal-to-noise ratio down to the lowest frequencies of interest.